How long do NOS 6L6gc power tubes last in a Mcintosh MC240 amp?

tomic601 is correct. the MC240 is pretty easy on tubes. not sure what speakers you are using but try both 8 and 4 ohm taps. usually one will sound a bit better. Also try some KT66s in the 240.
not sure if your 240 is original or not, but greatest improvement you can make is to add a film cap to the 100uf electrolytic cap in the feedback circuit.
Takes longer to remove the bottom cover than it does to solder in the cap, one for each channel.

Thanks @johnss and @tomic601, I have Klipsch heritage Belle speakers, and just ordered replacement tubes from Brent Jessee.

Garth - I bet that sounds fantastic!!!! Belles are really nice !

>> greatest improvement you can make is to add a film cap to the 100uf electrolytic cap in the feedback circuit.

Just finished restoring my MC240 and noticed the above comment from 2020. Can you give a couple more details?

-- What value of film cap, and how wired?

-- What's the goal here?

Thanks!

Phil Nelson (philsoldradios)

https://antiqueradio.org/index.html

Phil - the smaller cap in film sounds better than the single larger cap alone. Like all things audio, some hear it, some don't… The rule of thumb is 10% of the larger cap, use same voltage rating !  My 240 is singing for her supper ;-)